A. Field of the Invention
The present invention relates to a fast zoom lens system and, more particularly, to a compact fast zoom lens system comprising three lens groups.
B. Description of the Related Art
Automatic compact cameras have recently been provided with zoom lenses. One such compact camera, a leaf shutter camera, must also have a low price because it is typically used by non-professional photographers. Although normally these types of cameras are not used by professionals, this has been changing in recent years. Unfortunately, however, leaf shutter cameras do not meet the requirements of professional photographers since they presently provide fast lenses having a short focus and zoom lenses having an F number of more than 3.5.
Zoom lens systems having two lens groups have been proposed for leaf shutter cameras. One problem with these systems is that if the F number increases, a spherical aberration and coma will increase as well. As a result, the zoom lens system is usually used only when conditions require a zoom ratio of about 2.0 times and an F number of less than 3.5. Thus, the zoom lens systems suffer from the fact that an F number will decrease in proportion to a focal length at a telephoto position.
Zoom lens systems for leaf shutter cameras, having three lens groups, have been disclosed in Japanese Laid-Open Patent Publication Nos. Hei 2-201410 and Hei 2-37317. In these systems, the first lens group has a negative refractive power, the second lens group has a positive lens group, and the third lens group has a negative refractive power. These zoom lens systems can have a high zoom ratio and a high F number. However, they are not suitable for a compact camera since they are large in size.
Zoom lens systems for leaf shutter cameras, having three lens groups, have also been disclosed in U.S. Pat. No. 4,854,682, U.S. Pat. No. 4,978,204 and U.S. Pat. No. 5,148,321. In these systems, the first and second lens groups have a positive refractive power, while the third lens group has a negative refractive power. These zoom lens systems are compact and have a high zoom ratio, but the F number is more than 3.5. Thus, these zoom lens systems fail to meet professional standards as well.